Osmotic uptake of water is the driving force of plant cell expansion. As water enters the cell, the protoplast expands but is restrained by the cell wall. Moreover, a rigid complex of cellulose microfibril polymers embedded in a glue-like matrix of pectins, hemicelluloses and proteins forms part of this wall in mature cells. It has long been thought that some “wall loosening” factor must be present which alters immature cell wall mechanical properties and allows it to undergo a process of elongation. McQueen-Mason et al., Plant Cell, Vol. 4, pp. 1425-1433 (1992) studied plant cell enlargement regulation by employing a reconstitution approach. The authors found that a crude protein extract from the cell walls of growing cucumber seedlings possess the ability to induce the extension of isolated cell walls. Sequential HPLC fractionation of the active wall extract revealed two proteins with molecular masses of 29 and 30 kD associated with the activity. Each protein, by itself, could induce wall extension without detectable hydrolytic breakdown of the wall and appeared to mediate “acid growth” responses of isolated walls and may catalyze plant cell wall extension by a novel biochemical mechanism.
Shcherban et al., Proc. Nat. Acad. Sci., USA, Vol. 92, pp. 9245-9249 (1995) isolated cDNA's encoding these two cucumber proteins and compared them to anonymous expressed sequence tags from various sources. Rice and Arabidopsis expansin cDNA were identified from these collections and showed at least four different expansin cDNA's in rice and six different expansin cDNA's in Arabidopsis. The authors concluded that expansin are highly conserved in size and sequence (60-87% amino acid identity and 75-95% similarity between any pairwise comparison) and that the multigene family formed before the evolutionary divergence between monocotyledons and dicotyledons. Shcherban et al. states that the high conservation of this mutligene family indicates that the mechanism by which expansin promotes cell wall extension tolerates little variation in protein structure.
Wang et al., Biotech. Lett., Vol. 16, No. 9, pp. 955-958 (1994) discovered two proteins in a Chinese medicinal cucumber, Trichosanthes kirilowii, which appear to be similar to the S1 and S2 proteins which demonstrate cell wall extension properties. Similar proteins were also found in growing tomato leaves (Keller et al., The Plant Journal, Vol. 8, No. 6, pp. 795-802 (1995)) and in oat coleoptile walls (Li et al., Planta, Vol. 191, pp. 349-356 (1993)).
Cosgrove et al., J. Exp. Botany, Vol. 45, Special Issue, pp.1711-1719 (1994) suggested that cooperative interactions between the expansin proteins and pectinases and cellulases may occur, wherein the enzymes modify the matrix so that other wall extension mechanisms may be more effective. Fry, Current Bioloqy, Vol. 4, No. 9 (1994) suggest that, in loosening cell walls, expansin seems unlikely to break cellulose-cellulose bonds as microfibrils remain intact during growth. Thus, the authors discount the observed breakage of hydrogen bonds in filter paper as a side issue and suggest that expansin may lengthen inter-microfibrillar tethers by causing hemicellulose chains to detach from cellulose microfibrils to allow extension.
Despite the pioneering work previously done in the area of cell wall extension and its causes, work related to the usefulness and operability of expansins is still in its infancy. Moreover, the sources of expansin up to now have been exclusively from plant origins, for which expression systems may not be optimal for large scale production. Accordingly, it would be valuable to have a ready source of expansin-like material which is capable of being produce in large quantities from organisms which are established high output producers of biological materials, such as fungi, bacteria or other well characterized microorganisms.